1. Field of the Invention
The invention relates to the output stage of an axial-flow turbine having high channel divergence, with a row of curved vanes and with a row of narrowed twisted blades.
2. Discussion of Background
Curved vanes are used, in particular, for reducing the secondary losses which occur as a result of the deflection of the boundary layers in the vanes.
Turbines with vanes curved only in the circumferential direction are known, for example, from DE-A-37 43 738. This shows and describes vanes, the curvature of which is directed, over the vane height, toward the pressure side of the vane which is, in each case, adjacent in the circumferential direction. This publication also discloses vanes, the curvature of which is directed, over the vane height, toward the suction side of the vane which is, in each case, adjacent in the circumferential direction. Consequently, both radial and circumferentially running boundary layer pressure gradients are to be reduced effectively and, therefore, the aerodynamic vane losses minimized. Irrespective of the side of the adjacent vane toward which the curvature of this known vane is directed, at all events said curvature runs exactly in the circumferential direction. This means that, in the case of the cylindrical vanes illustrated, at least their leading edges lie in the same axial plane over the vane height.
Turbines having vanes curved in the axial direction and in the circumferential direction are known, for example, from DE-A-42 28 879. A fixed vane cascade is arranged upstream of the blade cascade. The number and the chord-to-pitch ratio of the vanes of said vane cascade are optimized in flow terms for full load. They give the flow the swirl necessary for entry into the blade cascade. The curvature of the vanes runs perpendicularly to the chord, this being achieved both in the circumferential direction and in the axial direction by means of a displacement of the profile sections. The curvature of the vanes is directed toward the pressure side of the vane which is, in each case, adjacent in the circumferential direction. This curvature is formed by a continuous arc which is at an acute angle to the vane carrier and to the hub. As a result of curvature perpendicular to the vane chord, the vane surface projected in the radial direction is greater than in the case of the known curvature in the circumferential direction. The radial force on the working medium is therefore increased; the latter is pressed onto the channel walls, with the result that the boundary layer thickness is reduced there.